Quinolone derivatives having a condensed pyridonecarboxylic acid skeleton are known as synthetic antimicrobial agents and to provide potent antimicrobial compounds on substitution of the 1-position thereof with a cyclopropyl group.
It is further known that the 1-cyclopropylquinolone derivatives having introduced a fluorine atom to the 2-position of the cyclopropyl group in a cis-configuration with the pyridonecarboxylic acid moiety also exhibit potent anti-microbial activity as disclosed in U.S. Pat. No. 4,871,852 which corresponds to JP-A-87-12760 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”). They are thought to have not only potent antimicrobial activity but improved safety. One example of the quinolone derivatives having a cis-fluorocyclopropyl group at the 1-position is shown below. 
Quinolone derivatives having a cis-halogeno-cyclopropyl group, inclusive of a cis-fluorocyclopropyl group, at the 1-position as stated above possess excellent properties in antimicrobial activity and safety. In these compounds, even when they have a substituent without stereo-isomerism at the 7-position of the pyridonecarboxylic acid moiety, the halogeno-cyclopropane ring of itself provides two enantiomers attributed to the static  steric relationship between the pyridonecarboxylic acid moiety and the halogen atom with respect to the cyclopropane ring as illustrated below; wherein R1, R2, A, X1, and X2 are hereinafter defined. These quinolone derivatives are applicable as medicines as long as they are racemates. On the other hand, when there is stereoisomerism at the 7-positioned substituent of the pyridonecarboxylic acid moiety, such quinolone derivatives contain four kinds of stereoisomers. With mixtures of the stereoisomers, it is difficult to specify the excellent species and to supply them as medicines.